The present invention relates to hydrodynamic bearings In such bearings, a rotating object such as a shaft is supported by a stationary bearing pad via a pressurized fluid such as oil, air or water. Hydrodynamic bearings take advantage of the fact that when the rotating object moves, it does not slide along the top of the fluid. Instead the fluid in contact with the rotating object adheres tightly to the rotating object, and motion is accompanied by slip or shear between the fluid particles through the entire height of the fluid film. Thus, if the rotating object and the contacting layer of fluid move at a velocity which is known, the velocity at intermediate heights of the fluid thickness decreases at a known rate until the fluid in contact with the stationary bearing pad adheres to the bearing pad and is motionless. When, by virtue of the load resulting from its support of the rotating object, the bearing pad is deflected at a small angle to the rotating member, the fluid will be drawn into the wedge-shaped opening, and sufficient pressure will be generated in the fluid film to support the load. This fact is utilized in thrust bearings for hydraulic turbines and propeller shafts of ships as well as in the conventional hydrodynamic journal bearing.
Both thrust bearings and radial or journal bearings normally are characterized by discrete shaft supporting pads spaced about an axis. The axis about which the pads are spaced generally corresponds to the longitudinal axis of the shaft to be supported for both thrust and journal bearings This axis may be termed the major axis.
In an ideal hydrodynamic bearing, the hydrodynamic wedge extends across the entire bearing pad face, the fluid film is just thick enough to support the load, the major axis of the bearing and the axis of the shaft are aligned, leakage of fluid from the ends of the bearing pad surface which are adjacent the leading and trailing edges is minimized, the fluid film is developed as soon as the shaft begins to rotate, and, in the case of thrust bearings, the bearing pads are equally loaded. While an ideal hydrodynamic bearing has yet to be achieved, a bearing which substantially achieves each of these objectives is said to be designed so as to optimize hydrodynamic wedge formation.
In known radial pad type bearings, it has heretofore been believed necessary to provide an accurately determined clearance between the bearing and the rotating object supported so as to allow the appropriate deflection of the bearing pads to form the hydrodynamic wedge. The requirement of close tolerances is particularly troublesome in the manufacture of gas lubricated bearings. Another problem with gas lubricated bearings is the breakdown of the fluid film at high speeds. These problems have limited the use of gas lubricated hydrodynamic bearings.
U.S. Pat. No. 3,107,955 to Trumpler discloses one example of a bearing having beam mounted bearing pads that displaces with a pivoting or swing-type motion about a center located in front of the pad surface. This bearing like many prior art bearings is based only on a two dimensional model of pad deflection. Consequently, optimum wedge formation is not achieved.
In the Hall patent, U.S. Pat. No. 2,137,487, there is shown a hydrodynamic movable pad bearing that develops its hydrodynamic wedge by sliding of its pad along spherical surfaces. In many cases the pad sticks and the corresponding wedge cannot be developed. In the Greene Patent, U.S. Pat. No. 3,930,691, the rocking is provided by elastomers that are subject to contamination and deterioration.
U.S. Pat. No. 4,099,799 to Etsion discloses a non-unitary cantilever mounted resilient pad gas bearing. The disclosed bearing employs a pad mounted on a rectangular cantilever beam to produce a lubricating wedge between the pad face and the rotating shaft. Both thrust bearings and radial or journal bearings are disclosed.
There is shown in the Ide patent, U.S. Pat. No. 4,496,251 a pad which deflects with web-like ligaments so that a wedge shaped film of lubricant is formed between the relatively moving parts.
U.S. Pat. No. 4,515,486 discloses hydrodynamic thrust and journal bearings comprising a number of bearing pads, each having a face member and a support member that are separated and bonded together by an elastomeric material.
U.S. Pat. No. 4,526,482 discloses hydrodynamic bearings which are primarily intended for process lubricated applications, i.e., the bearing is designed to work in a fluid. The hydrodynamic bearings are formed with a central section of the load carrying surface that is more compliant than the remainder of the bearings such that they will deflect under load and form a pressure pocket of fluid to carry high loads.
It has also been noted in Ide U.S. Pat. No. 4,676,668, that bearing pads may be spaced from the support member by at least one leg which provides flexibility in three directions. To provide flexibility in the plane of motion, the legs are angled inward to form a conical shape with the apex of the cone or point of intersection in front of the pad surface. Each leg has a section modulus that is relatively small in the direction of desired motion to permit compensation for misalignments. These teachings are applicable to both journal and thrust bearings. While the disclosure of this patent represents a significant advance in the art, it has some shortcomings. One such shortcoming is the rigidity of the support structure and bearing pad which inhibits deformation of the pad surface. Further, the bearing construction is not unitary.
The last two patents are of particular interest because they demonstrate that despite the inherent and significant differences between thrust and journal bearings, there is some conceptual similarity between hydrodynamic journal bearings and hydrodynamic thrust bearings.
This application relates in part to hydrodynamic thrust bearings. When the hydrodynamic wedge in such bearings is optimized, the load on each of the circumferentially spaced bearings is substantially equal.
Presently, the most widely used hydrodynamic thrust bearing is the so-called Kingsbury shoe-type bearing. The shoe-type Kingsbury bearing is characterized by a complex structure which includes pivoted shoes, a thrust collar which rotates with the shaft and applies load to the shoes, a base ring for supporting the shoes, a housing or mounting which contains and supports the internal bearing elements, a lubricating system and a cooling system. As a result of this complex structure, Kingsbury shoe-type bearings are typically extraordinarily expensive.
An alternative to the complex Kingsbury shoe-type bearing is a unitary pedestal bearing which has been employed in, among other things, deep well pumps. This relatively simple structure is typically formed by sand casting or some other crude manufacturing technique. The bearing is structurally characterized by a flat base having a thick inner circumferential projection, a plurality of rigid pedestals extending transversely from the base and a thrust pad centered on each rigid pedestal.
The present inventor has also discovered that the center pivot nature of both the known rigid pedestal bearing shown and the Kingsbury shoe-type bearing contributes to bearing inefficiency. It should also be noted that, because of their rigid center pivots, neither the Kingsbury shoe-type bearings nor the pedestal bearing shown can deflect with six degrees of freedom to optimize wedge formation. Thus, while, in some instances, the prior art bearings are capable of movement with six degrees of freedom, because the bearings are not modeled based upon or designed for six degrees of freedom, the resulting performance capabilities of these bearings are limited.
Known hydrodynamic bearings, whether radial or thrust, all suffer (to varying degrees) from a number of problems. For example, prior art hydrodynamic bearings often suffer from fluid leakage which causes breakdown of the fluid film. In radial bearings, the leakage primarily occurs at the axial ends of the bearing pad surface. In thrust bearings, the leakage primarily occurs at the outer circumferential periphery of the pad surface as a result of centrifugal forces action on the fluid. What fluid leakage can be significantly reduced by supporting the bearing pads for optimum deflection, known bearing which have discrete pads (and consequently spaces in the bearing surface) inherently have some fluid leakage.
Further, it is difficult to optimize the quality of the finish of the bearing pad surface of conventional bearings having separate pads. The quality of the finish is important to ensure uniform fluid film distribution.